package com.carl.math;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class question_02 {
    public static void main(String[] args) throws IOException {
        //差值
        BufferedReader bf = new BufferedReader(new InputStreamReader(System.in));
        String[] arg = bf.readLine().split(" ");
        int a = Integer.parseInt(arg[0]),b=Integer.parseInt(arg[1]);
        int c = Integer.parseInt(arg[2]),d = Integer.parseInt(arg[3]);
        System.out.printf("result = %d",a*b-c*d);


    }
}
